1. Field of the Invention
The present invention relates to antenna switching diversity reception technology used in a direct sequence spread spectrum communication system.
2. Description of the Related Art
In wireless communication using digital portable telephones and automobile telephones and the like, due to the fading attributable to the movement of mobile communication terminals and to a multipath environment, the receive signal gives rise to a level fluctuation of several tens of dB and to phase fluctuation, and leads to an increase of encoding errors. For this reason, as technology to compensate for a drop in the reception level, the technology of post-wave detection switching diversity, antenna switching diversity, and the like, has been proposed. Post-wave detection diversity is a method that provides, inside receiving equipment, two or more systems, of antennas, and receivers that demodulate received radio waves and regenerate the received data, carries out data regeneration using each receiving system, and utilizes the data regenerated by the receiving system with the highest reception level. However, because post-wave detection switching diversity requires two or more receiving systems, there are the disadvantages that the construction of the receiving device becomes complex, and the manufacturing costs become high. For this reason, the system is not suitable for digital portable telephones which require a low cost and a simple construction.
Accordingly, antenna switching diversity has been proposed as technology to realize diversity at a low cost and with a simple construction. One example of antenna switching diversity receiving technology is explained with reference to FIG. 6. The diversity receiving device shown in this same figure is that which was disclosed in Japanese Patent Application Laid-open No. 2000-295150. In the same figure, reference numerals 27, 28 are antennas, 29 is an antenna switching device that switches the antennas, 30 is a receiving portion that receives and demodulates radio waves from a base station via antenna 27 or 28 and outputs the received data and has the capability to detect the strength of the receive signal. 31 is an antenna selection means that controls the antenna switching device 29 in response to the level of the receive signal detected by the receiving portion 30. The antenna selection means 31 comprises a low-pass filter 32 that filters the receive signal strength, a threshold generating means 33 that outputs the appropriate switching threshold with respect to the average receive signal strength, and a comparison means 34 that compares the switching threshold obtained from the threshold generating means 33 and the receive signal strength, and controls the antenna switching device 29 so as to switch the antennas when the receive signal strength has fallen below the switching threshold.
A low-pass filter 32 is used that has a cut-off frequency at the level of a Doppler shift frequency, which is produced in a received wave by a moving speed at a level equivalent to a walking speed. The switching threshold output by the threshold generating means 33 is set in response to the value of the receive signal strength filtered by the low-pass filter 32, and is set in a range of a prescribed upper limit value and lower limit value. By means of the above-mentioned construction, the switching threshold can be set in an appropriate range, even when the fading speed is slow, and a reduction in the benefits of diversity reception can be suppressed.
However, in the above-mentioned construction, there is the problem that the receive signal strength is judged to be high and antenna switching is carried out, even when the noise level contained in the receive signal is high, because it is not possible to judge the noise level of an interference wave, or the like, contained in a receive signal. This kind of problem becomes still more serious because mutual interference easily occurs in frequency bands, in which many communication systems are combined, such as in the ISM band (2.4 GHz band) used in the low power data communication system of a wireless LAN and the like. Furthermore, with antenna switching diversity, because excessive switching noise is generated at the time of switching the antenna connection in the antenna switching device, the deterioration of the signal-to-noise ratio (SNR), when antenna switching is carried out at the time of sending and receiving data, becomes a problem.